The present invention relates to a new active oxygen scavenger. More particularly, it relates to an active oxygen scavenger comprising as an active ingredient a tetrazolylalkoxy-dihydrocarbostyril compound of the formula (I) 
wherein R is a cycloalkyl group, A is a lower alkylene group, and the bond between 3- and 4-positions of the carbostyril nucleus means a single bond or a double bond, or a salt thereof, and an agent for the prevention or treatment of an acute cerebral infarction.
Oxygen is essential for a living body in order to maintain life, for example, to keep normal energy production and metabolism. The oxygen may be changed into so-called active oxygen species such as an oxygen anion radical, a peroxyl radical, a hydroxyl radical by reactions in the energy producing system, an oxygen reaction, a reaction with ultraviolet or X-ray, etc. The active oxygen species are useful for living body, e.g. for oxygen-addition enzyme or antibacterial action of leukocytes, but on the other hand, the species promote peroxidation of unsaturated fatty acids such as oleic acid, linolenic acid, or arachidonic acid which form phospholipids in living membranes. The lipid peroxides induce generation of alkoxy radicals and hydroxyl radicals like the above active oxygen species, and attack the living membranes which results in membrane injury or inactivation of various enzymes. (cf. xe2x80x9cTAISHA (Metabolism)xe2x80x9d 15(10), 1978, xe2x80x9cSpecial Feature of Active Oxygenxe2x80x9d) However, the living body includes various enzymes such as superoxide dimutase (SOD), catalase, glutathione peroxidase, which participate in metabolic inactivation of active oxygen species as mentioned above, and also includes vitamins such as xcex1-tocopherol (vitamin E) which have an antioxidation activity. The living body is usually maintained in normal state by the actions of these enzymes and vitamins. However, there may happen deficiency in the biophylactic mechanism by these enzymes and vitamins or excess generation of active oxygen species which induce loss of control of the biophylactic mechanism or production and deposition of lipid peroxides. When deficiency of biophylactic mechanism occurs, peroxidation reactions will progress into chain reaction, which induce serious disorders, for example acute cerebral infarction.
The present inventors have intensively studied to develop a new medicament for scavenging active oxygen and found that the carbostyril compound of the formula (I) or a salt thereof as mentioned above, particularly 6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]-3,4-dihydrocarbostyril or a salt thereof, have an activity of scavenging active oxygen species and hence are useful as an active oxygen scavenger, and then the present invention has been accomplished.